Heat-treating apparatus



Patented Aug. 24, 1943 2,327,804 HEAT-TREATING APPARATUS Roland P. Koehring, Dayton,

General Motors Corporation, corporation of Delaware Ohio, assignor to Detroit, Mich, a

Original application May 9, 1938, Serial No.

206,727. Divided and this application February 28, 1941, Serial No. 381,184

1 Claim.

This invention relates to heat treating furnaces and is more particularly concerned with furnaces in which temperatures are equalized due to a circulation of the atmosphere therein as caused by the injection of gases at a high velocity.

This application is a division of my copending application 206,727 filed May 9, 1938, said application now being matured into Patent No. 2,236,966, granted April 8, 1941.

An object of the invention is to provide a furnace in which a controlled atmosphere is injected therein at a high velocity through restricted passages within the furnace whereby the furnace atmosphere is circulated due to a Venturi eifect created by the injection of the atmosphere whereby the temperature within the furnace is substantially equalized. In carrying out the above object it is a further object to provide a plurality of tubular heating elements through which gases may be injected which elements act as Venturi tubes.

A further object in some cases is to form restricted throat portions within the tubes either by forming the tubes to the desired shape or by insertion of refractory throats therein whereby the entrainment of furnace atmosphere by the atmosphere injected into the tubes is increased.

Further objects and advantages of the present invention will be apparent from the following description, reference being had to the accompanying drawing wherein preferred embodiments of the present invention are clearly shown.

In the drawing:

Fig. 1 is a cross sectional view of one type of heat treating furnace.

Fig. 2 is a fragmentary view showing the heating elements as taken on line 2-2 of Fig. 1.

Fig. 3 is an enlarged view of a heating element including a refractory throat therein and Fig. 4 is a view of another form of heating element wherein the throat is formed by the metal of the element.

In the usual type of controlled atmosphere furnace, the controlled atmosphere such as, hydrogen, nitrogen, carbon monoxide, carbon dioxide, etc. or mixtures thereof, is admitted into the heating chamber at slow velocity and under a pressure slightly in excess of atmospheric pressure. This pressure ratio prevents the ingress of any substantial quantity of atmospheric air and thereby maintains the controlled atmosphere within the furnace. In such controlled atmosphere furnaces, and in fact in any type of heat treating furnace, the charge to be heated is usually not heated to any degree of uniformity due to the poor circulation of atmosphere within thefurnace. This is explained by the fact that the gases in the upper part of the furnace are usually hotter than the gases in the lower part thereof, and as a result the upper portion of the charge is usually heated to a higher temperature than the lower portion thereof. Similarly when a number of small charges are placed within the furnace those adjacent the radiating surfaces of the heating elements are heated at a higher rate and to a higher temperature than the charges which are shielded from the heating surfaces by the remainder of the charge. Furnaces of this 7 character are used in the heat treating of metallic articles such as dies and other tools and are likewise used in sintering powdered metal briquettes and the like.

In an eil'ort to overcome the aforementioned difllculties, circulating fans have been utilized within heat treating fumacesjo cause a'circulation of the atmosphere therein, but as yet it has been found impractical to utilize such fans in furnaces wherein the temperature is'in excess of 1600 F. In the sintering furnaces hereinbefore mentioned temperatures are normally maintained between 1800 and 2300 F., and therefore the use of circulating fans is impractical.

The present invention is particularly concerned with providing uniform temperatures throughout a heat treating furnace without the use of a fan and is preferably used in connection with controlled atmosphere furnaces wherein it is necessary to introduce a controlled atmosphere gas to replenish and maintain the atmosphere within the furnace.

Referring particularly to Fig. 1, a cross sectional view of one type of furnace is shown wherein a frame 20 supports an enclosed covering 22 for a muille chamber 24. Chamber 24 has a longitudinal support 2. therein for a conveyor II which passes through the furnace for carrying articles to be sintered. Adjacent the walls of the chamber 24 are a plurality of tubular heating elements ll. Aligned with each of the elements I0 is an injection port or jet 32 which is supported by and fed from a manifold II. Manifold 3 3 carries a controlled atmosphere gas under a desired pressure as well as in excess of atmospheric pressure. The jets 32 inject the.

controlled atmosphere into the furnace through the tubular elements 30 whereby furnace atmosphere is entrained due to a Venturi effect and circulated up through the elements ll wherein it is heated during movement thereof. In this manner, circulation of the furnace atmosphere is accomplished whereby the furnace temperature is maintained substantially equal throughout the chamber 26.

In order to increase the Venturi effect and thereby enhance the circulation of gas in the iumace, it may be desirable in some cases to place throats 3 within the tubular elements 8@. Throats 34 are preferablyiormed of a refractory material and are shaped at the inside like aventuri tube to provide maximum circulation of atmosphere.

Fig. 4 shows another embodiment of heating element as wherein the entire tube is shaped like a venturi but wherein the thickness of the wall of the tube varies for maintaining a con= I stant cross sectional area throughout the length of the tube. In this manner, the electrical characteristic of the element are constant through out its length but the shape of the tube increases the Venturi efiect created by the injection of gas or controlled atmosphere therethrough.

In all the embodiments an entrainment oi the furnace atmosphere is provided which causes circulation of the furnace atmosphere and equalization of furnace temperatures.

asaasoa While the embodiments of the present invention as herein disclosed, constitute preferred forms, it is to be understood that other forms might be adopted, all coming within the scope alignedwith said resistance elements for iniecting controlled atmosphere into the furnace at high velocity and into said elements whereby the furnace atmosphere is entrained by said injected atmosphere through the cooperation of said throat portions for causing circulation of the furnace atmosphere and thereby equalizing temperatures within the furnace. 

